That’s not a React error. That’s based on your module loader (webpack, I’m guessing?) treating the jQuery plugin as if it were an ES2015 module. Try importing your plugin in the CommonJS style. In other words, replace:

import mCustomScrollbar from 'malihu-custom-scrollbar-plugin';

with:

var mCustomScrollbar = require('malihu-custom-scrollbar-plugin');

If that doesn’t work, you will need to share more info about your module loader and its configuration.

That said, as Toby mentioned, trying to combine jQuery plugins with React components is often unreliable and tricky, so proceed with caution.